An in-depth exploration of any underground formation containing hydrocarbons is a necessary prerequisite to the extraction of hydrocarbons from the formation.
In order to proceed with that in-depth exploration, it is known to drill an exploration well and insert a series of instruments into said exploration well making it possible to perform in situ measurements: pressure measurement, temperature measurement, sample withdrawal, etc. It is also known to incorporate the set of measuring instruments into a downhole well tool assembled on a cable (“wireline formation tester”) and adapted to be lowered into the well to determine the profiles of various parameters along the well.
In particular, the pressure measurements are used to determine the mobility of the fluids contained in the underground formation and the permeability of the underground formation. In general, the pressure is measured by locally imposing a vacuum through fluid suction in a test chamber provided with a piston until the filter cake of the well is broken, then allowing the system to return to equilibrium and measuring the evolution of the pressure during the return to equilibrium.
It is in this way that the company Schlumberger developed several generations of downhole well tools making it possible in particular to perform pressure measurements. First, the RFT (repeat formation tester) tool comprises two test chambers, the first operating at a fixed rate Q1 and the second operating at a fixed rate Q2 that is twice the rate Q1. A unique measuring sequence is carried out by suctioning the fluid successively in both chambers. This device does not make it possible to perform several successive measurement sequences (pre-tests) at a same position along the well. Furthermore, the suctioned fluid flow rate is not adjustable, but the necessary rate varies greatly depending on the characteristics of the underground formation.
Moreover, the MDT (modular formation dynamics tester tool) is equipped with a single test chamber provided with a hydraulic control motor. Lastly, the XPT tool (express pressure tool) comprises a test chamber provided with an electric control motor with a worm screw. These tools allow monitoring of the flow rate during the pre-tests, but the precision of the flow rate or the extent of the achievable flow rate range remain unsatisfactory.
Furthermore, one problem related to these tools is that when several pre-tests are linked to a same position along the well, the evolution of the pressure during the return toward equilibrium necessarily differs from one pre-test to the next, due to the variation of the fluid volume in the instrument. The duration of the transitional state increases during successive pre-tests. Thus, it is necessary to acquire the pressure over a very long time period in order to be able to perform statistical processing of the results, as well as to verify whether the measurements are consistent with one another or if the measurements may be inconsistent and therefore not significant, which is the case of low-permeability underground formations. In fact, in low-permeability underground formations, overload phenomena occur, i.e. the mud in the well tends to penetrate the underground formation due to the small difference between the permeability of the filter cake and the permeability of the formation (the filter cake being correlatively not very sealed relative to the underground formation).
In other words, the tools of the state of the art do not make it possible to quickly identify situations in which the permeability of the underground formation is too low to allow a significant pressure measurements; and they do not make it possible to quickly carry out repeated pre-tests in order to obtain truly representative pressure data.
There is therefore a major need to develop a method and a device making it possible to perform pressure measurements in an underground formation more quickly, simply and reliably than with the methods and devices of the state of the art.